The invention relates to a vapor guard system for a servomotor, and more particularly to a system which prevents vapors from reaching the servomotor at any time, such vapors being present at a pressure source for the servomotor. In the disclosed embodiments of the invention the servomotor is a vacuum suspended power brake booster powered by vacuum from the intake of the automobile engine and air at atmospheric pressure. The invention can also be used with other types of servomotors having different functions but having one or more parts which may be adversely affected by vapors from a pressure source. It could, for example, be used in a servomotor in a positive pressure system in which vapors are found which possibly cause damage to the servomotor.
The invention is an improvement over the vapor guard system disclosed and claimed in U.S. Pat. No. 4,109,464, issued Aug. 29, 1978 and assigned to the common assignee.
The type of vacuum suspended power brake booster systems in use for many years in vehicles has included a brake booster having a diaphragm as part of the power wall and dividing the booster housing into a vacuum chamber and a variable pressure chamber. When the booster is inactive, both chambers are vacuum charged. Upon booster actuation the connection between the two chambers is closed and atmospheric air pressure is controllably introduced into the variable pressure chamber, setting up a pressure differential across the power wall and actuating the booster. A check valve is provided as a part of the booster and provides a connection between the vacuum conduit and the booster vacuum chamber. The other end of the vacuum conduit is tapped into a portion of the vehicle internal combustion engine where intake vacuum is present. This may be at the base of the carburetor or in a portion of the intake manifold downstream of the carburetor, for example. In certain installations it has been found that fuel vapor is present at the point where the vacuum conduit is tapped in and, under certain relatively rare conditions, can be pulled into the vacuum chamber of the booster and condensed. One such condition has been found to occur in extremely cold ambient temperatures which cause the check valve not to fully seal the vacuum conduit relative to the vacuum chamber when the absolute pressure in the vacuum chamber is lower than the absolute pressure in the engine intake acting as a vacuum source. This can occur upon full throttle opening or when the engine is not running. If a sufficient concentration of fuel vapor is located at the vacuum conduit tap-in point on the engine when this occurs, it will be entrained in the flow of air that moves from the engine through the conduit toward the vacuum chamber. A concentration of fuel vapor and condensed fuel in the vacuum chamber can adversely affect materials made of synthetic or natural rubbers. The power diaphragm is usually made of such a material. It is therefore desirable to prevent any fuel vapor from entering the brake booster through the vacuum conduit if the various circumstances occur together in an installation which would tend to introduce the fuel vapor into the booster.
The vapor guard system disclosed in U.S. Pat. No. 4,109,464 includes a filter in the vacuum conduit between the internal combustion engine source of vacuum and the brake booster check valve so that all fluid flowing in either direction between the brake booster and the engine vacuum source must pass through the filter. The filter is provided with a material which will trap or adsorb the fuel vapor that is entrained in any air flow from the internal combustion engine toward the brake booster. When the air flow returns to the normal flow pattern, which is the predominant condition where there is any flow at all, the fuel vapor so trapped or adsorbed will be released or desorbed and will be returned to the engine with the air flowing from the booster toward the engine. Thus at no time does the deleterious vapor come into the presence of any part of the brake booster which can be adversely affected by it, insofar as transmission of such vapor through the vacuum conduit connecting the booster to the internal combustion engine is concerned. The material used in the filter to adsorb the fuel vapors may be activated charcoal. Other suitable materials may also be utilized.